1 Dynamic DMA mapping using the generic device
2 ============================================
3 4 James E.J. Bottomley <James.Bottomley@HansenPartnership.com>
5 6This document describes the DMA API. For a more gentle introduction
7of the API (and actual examples) see
8Documentation/DMA-API-HOWTO.txt.
9 10This API is split into two pieces. Part I describes the API. Part II
11describes the extensions to the API for supporting non-consistent
12memory machines. Unless you know that your driver absolutely has to
13support non-consistent platforms (this is usually only legacy
14platforms) you should only use the API described in part I.
15 16Part I - dma_ API
17-------------------------------------
18 19To get the dma_ API, you must #include <linux/dma-mapping.h>
20 21 22Part Ia - Using large dma-coherent buffers
23------------------------------------------
24 25void *
26dma_alloc_coherent(struct device *dev, size_t size,
27 dma_addr_t *dma_handle, gfp_t flag)
28 29Consistent memory is memory for which a write by either the device or
30the processor can immediately be read by the processor or device
31without having to worry about caching effects. (You may however need
32to make sure to flush the processor's write buffers before telling
33devices to read that memory.)
34 35This routine allocates a region of <size> bytes of consistent memory.
36It also returns a <dma_handle> which may be cast to an unsigned
37integer the same width as the bus and used as the physical address
38base of the region.
39 40Returns: a pointer to the allocated region (in the processor's virtual
41address space) or NULL if the allocation failed.
42 43Note: consistent memory can be expensive on some platforms, and the
44minimum allocation length may be as big as a page, so you should
45consolidate your requests for consistent memory as much as possible.
46The simplest way to do that is to use the dma_pool calls (see below).
47 48The flag parameter (dma_alloc_coherent only) allows the caller to
49specify the GFP_ flags (see kmalloc) for the allocation (the
50implementation may choose to ignore flags that affect the location of
51the returned memory, like GFP_DMA).
52 53void *
54dma_zalloc_coherent(struct device *dev, size_t size,
55 dma_addr_t *dma_handle, gfp_t flag)
56 57Wraps dma_alloc_coherent() and also zeroes the returned memory if the
58allocation attempt succeeded.
59 60void
61dma_free_coherent(struct device *dev, size_t size, void *cpu_addr,
62 dma_addr_t dma_handle)
63 64Free the region of consistent memory you previously allocated. dev,
65size and dma_handle must all be the same as those passed into the
66consistent allocate. cpu_addr must be the virtual address returned by
67the consistent allocate.
68 69Note that unlike their sibling allocation calls, these routines
70may only be called with IRQs enabled.
71 72 73Part Ib - Using small dma-coherent buffers
74------------------------------------------
75 76To get this part of the dma_ API, you must #include <linux/dmapool.h>
77 78Many drivers need lots of small dma-coherent memory regions for DMA
79descriptors or I/O buffers. Rather than allocating in units of a page
80or more using dma_alloc_coherent(), you can use DMA pools. These work
81much like a struct kmem_cache, except that they use the dma-coherent allocator,
82not __get_free_pages(). Also, they understand common hardware constraints
83for alignment, like queue heads needing to be aligned on N-byte boundaries.
84 85 86 struct dma_pool *
87 dma_pool_create(const char *name, struct device *dev,
88 size_t size, size_t align, size_t alloc);
89 90The pool create() routines initialize a pool of dma-coherent buffers
91for use with a given device. It must be called in a context which
92can sleep.
93 94The "name" is for diagnostics (like a struct kmem_cache name); dev and size
95are like what you'd pass to dma_alloc_coherent(). The device's hardware
96alignment requirement for this type of data is "align" (which is expressed
97in bytes, and must be a power of two). If your device has no boundary
98crossing restrictions, pass 0 for alloc; passing 4096 says memory allocated
99from this pool must not cross 4KByte boundaries.
100 101 102 void *dma_pool_alloc(struct dma_pool *pool, gfp_t gfp_flags,
103 dma_addr_t *dma_handle);
104 105This allocates memory from the pool; the returned memory will meet the size
106and alignment requirements specified at creation time. Pass GFP_ATOMIC to
107prevent blocking, or if it's permitted (not in_interrupt, not holding SMP locks),
108pass GFP_KERNEL to allow blocking. Like dma_alloc_coherent(), this returns
109two values: an address usable by the cpu, and the dma address usable by the
110pool's device.
111 112 113 void dma_pool_free(struct dma_pool *pool, void *vaddr,
114 dma_addr_t addr);
115 116This puts memory back into the pool. The pool is what was passed to
117the pool allocation routine; the cpu (vaddr) and dma addresses are what
118were returned when that routine allocated the memory being freed.
119 120 121 void dma_pool_destroy(struct dma_pool *pool);
122 123The pool destroy() routines free the resources of the pool. They must be
124called in a context which can sleep. Make sure you've freed all allocated
125memory back to the pool before you destroy it.
126 127 128Part Ic - DMA addressing limitations
129------------------------------------
130 131int
132dma_supported(struct device *dev, u64 mask)
133 134Checks to see if the device can support DMA to the memory described by
135mask.
136 137Returns: 1 if it can and 0 if it can't.
138 139Notes: This routine merely tests to see if the mask is possible. It
140won't change the current mask settings. It is more intended as an
141internal API for use by the platform than an external API for use by
142driver writers.
143 144int
145dma_set_mask(struct device *dev, u64 mask)
146 147Checks to see if the mask is possible and updates the device
148parameters if it is.
149 150Returns: 0 if successful and a negative error if not.
151 152int
153dma_set_coherent_mask(struct device *dev, u64 mask)
154 155Checks to see if the mask is possible and updates the device
156parameters if it is.
157 158Returns: 0 if successful and a negative error if not.
159 160u64
161dma_get_required_mask(struct device *dev)
162 163This API returns the mask that the platform requires to
164operate efficiently. Usually this means the returned mask
165is the minimum required to cover all of memory. Examining the
166required mask gives drivers with variable descriptor sizes the
167opportunity to use smaller descriptors as necessary.
168 169Requesting the required mask does not alter the current mask. If you
170wish to take advantage of it, you should issue a dma_set_mask()
171call to set the mask to the value returned.
172 173 174Part Id - Streaming DMA mappings
175--------------------------------
176 177dma_addr_t
178dma_map_single(struct device *dev, void *cpu_addr, size_t size,
179 enum dma_data_direction direction)
180 181Maps a piece of processor virtual memory so it can be accessed by the
182device and returns the physical handle of the memory.
183 184The direction for both api's may be converted freely by casting.
185However the dma_ API uses a strongly typed enumerator for its
186direction:
187 188DMA_NONE no direction (used for debugging)
189DMA_TO_DEVICE data is going from the memory to the device
190DMA_FROM_DEVICE data is coming from the device to the memory
191DMA_BIDIRECTIONAL direction isn't known
192 193Notes: Not all memory regions in a machine can be mapped by this
194API. Further, regions that appear to be physically contiguous in
195kernel virtual space may not be contiguous as physical memory. Since
196this API does not provide any scatter/gather capability, it will fail
197if the user tries to map a non-physically contiguous piece of memory.
198For this reason, it is recommended that memory mapped by this API be
199obtained only from sources which guarantee it to be physically contiguous
200(like kmalloc).
201 202Further, the physical address of the memory must be within the
203dma_mask of the device (the dma_mask represents a bit mask of the
204addressable region for the device. I.e., if the physical address of
205the memory anded with the dma_mask is still equal to the physical
206address, then the device can perform DMA to the memory). In order to
207ensure that the memory allocated by kmalloc is within the dma_mask,
208the driver may specify various platform-dependent flags to restrict
209the physical memory range of the allocation (e.g. on x86, GFP_DMA
210guarantees to be within the first 16Mb of available physical memory,
211as required by ISA devices).
212 213Note also that the above constraints on physical contiguity and
214dma_mask may not apply if the platform has an IOMMU (a device which
215supplies a physical to virtual mapping between the I/O memory bus and
216the device). However, to be portable, device driver writers may *not*
217assume that such an IOMMU exists.
218 219Warnings: Memory coherency operates at a granularity called the cache
220line width. In order for memory mapped by this API to operate
221correctly, the mapped region must begin exactly on a cache line
222boundary and end exactly on one (to prevent two separately mapped
223regions from sharing a single cache line). Since the cache line size
224may not be known at compile time, the API will not enforce this
225requirement. Therefore, it is recommended that driver writers who
226don't take special care to determine the cache line size at run time
227only map virtual regions that begin and end on page boundaries (which
228are guaranteed also to be cache line boundaries).
229 230DMA_TO_DEVICE synchronisation must be done after the last modification
231of the memory region by the software and before it is handed off to
232the driver. Once this primitive is used, memory covered by this
233primitive should be treated as read-only by the device. If the device
234may write to it at any point, it should be DMA_BIDIRECTIONAL (see
235below).
236 237DMA_FROM_DEVICE synchronisation must be done before the driver
238accesses data that may be changed by the device. This memory should
239be treated as read-only by the driver. If the driver needs to write
240to it at any point, it should be DMA_BIDIRECTIONAL (see below).
241 242DMA_BIDIRECTIONAL requires special handling: it means that the driver
243isn't sure if the memory was modified before being handed off to the
244device and also isn't sure if the device will also modify it. Thus,
245you must always sync bidirectional memory twice: once before the
246memory is handed off to the device (to make sure all memory changes
247are flushed from the processor) and once before the data may be
248accessed after being used by the device (to make sure any processor
249cache lines are updated with data that the device may have changed).
250 251void
252dma_unmap_single(struct device *dev, dma_addr_t dma_addr, size_t size,
253 enum dma_data_direction direction)
254 255Unmaps the region previously mapped. All the parameters passed in
256must be identical to those passed in (and returned) by the mapping
257API.
258 259dma_addr_t
260dma_map_page(struct device *dev, struct page *page,
261 unsigned long offset, size_t size,
262 enum dma_data_direction direction)
263void
264dma_unmap_page(struct device *dev, dma_addr_t dma_address, size_t size,
265 enum dma_data_direction direction)
266 267API for mapping and unmapping for pages. All the notes and warnings
268for the other mapping APIs apply here. Also, although the <offset>
269and <size> parameters are provided to do partial page mapping, it is
270recommended that you never use these unless you really know what the
271cache width is.
272 273int
274dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
275 276In some circumstances dma_map_single and dma_map_page will fail to create
277a mapping. A driver can check for these errors by testing the returned
278dma address with dma_mapping_error(). A non-zero return value means the mapping
279could not be created and the driver should take appropriate action (e.g.
280reduce current DMA mapping usage or delay and try again later).
281 282 int
283 dma_map_sg(struct device *dev, struct scatterlist *sg,
284 int nents, enum dma_data_direction direction)
285 286Returns: the number of physical segments mapped (this may be shorter
287than <nents> passed in if some elements of the scatter/gather list are
288physically or virtually adjacent and an IOMMU maps them with a single
289entry).
290 291Please note that the sg cannot be mapped again if it has been mapped once.
292The mapping process is allowed to destroy information in the sg.
293 294As with the other mapping interfaces, dma_map_sg can fail. When it
295does, 0 is returned and a driver must take appropriate action. It is
296critical that the driver do something, in the case of a block driver
297aborting the request or even oopsing is better than doing nothing and
298corrupting the filesystem.
299 300With scatterlists, you use the resulting mapping like this:
301 302 int i, count = dma_map_sg(dev, sglist, nents, direction);
303 struct scatterlist *sg;
304 305 for_each_sg(sglist, sg, count, i) {
306 hw_address[i] = sg_dma_address(sg);
307 hw_len[i] = sg_dma_len(sg);
308 }
309 310where nents is the number of entries in the sglist.
311 312The implementation is free to merge several consecutive sglist entries
313into one (e.g. with an IOMMU, or if several pages just happen to be
314physically contiguous) and returns the actual number of sg entries it
315mapped them to. On failure 0, is returned.
316 317Then you should loop count times (note: this can be less than nents times)
318and use sg_dma_address() and sg_dma_len() macros where you previously
319accessed sg->address and sg->length as shown above.
320 321 void
322 dma_unmap_sg(struct device *dev, struct scatterlist *sg,
323 int nhwentries, enum dma_data_direction direction)
324 325Unmap the previously mapped scatter/gather list. All the parameters
326must be the same as those and passed in to the scatter/gather mapping
327API.
328 329Note: <nents> must be the number you passed in, *not* the number of
330physical entries returned.
331 332void
333dma_sync_single_for_cpu(struct device *dev, dma_addr_t dma_handle, size_t size,
334 enum dma_data_direction direction)
335void
336dma_sync_single_for_device(struct device *dev, dma_addr_t dma_handle, size_t size,
337 enum dma_data_direction direction)
338void
339dma_sync_sg_for_cpu(struct device *dev, struct scatterlist *sg, int nelems,
340 enum dma_data_direction direction)
341void
342dma_sync_sg_for_device(struct device *dev, struct scatterlist *sg, int nelems,
343 enum dma_data_direction direction)
344 345Synchronise a single contiguous or scatter/gather mapping for the cpu
346and device. With the sync_sg API, all the parameters must be the same
347as those passed into the single mapping API. With the sync_single API,
348you can use dma_handle and size parameters that aren't identical to
349those passed into the single mapping API to do a partial sync.
350 351Notes: You must do this:
352 353- Before reading values that have been written by DMA from the device
354 (use the DMA_FROM_DEVICE direction)
355- After writing values that will be written to the device using DMA
356 (use the DMA_TO_DEVICE) direction
357- before *and* after handing memory to the device if the memory is
358 DMA_BIDIRECTIONAL
359 360See also dma_map_single().
361 362dma_addr_t
363dma_map_single_attrs(struct device *dev, void *cpu_addr, size_t size,
364 enum dma_data_direction dir,
365 struct dma_attrs *attrs)
366 367void
368dma_unmap_single_attrs(struct device *dev, dma_addr_t dma_addr,
369 size_t size, enum dma_data_direction dir,
370 struct dma_attrs *attrs)
371 372int
373dma_map_sg_attrs(struct device *dev, struct scatterlist *sgl,
374 int nents, enum dma_data_direction dir,
375 struct dma_attrs *attrs)
376 377void
378dma_unmap_sg_attrs(struct device *dev, struct scatterlist *sgl,
379 int nents, enum dma_data_direction dir,
380 struct dma_attrs *attrs)
381 382The four functions above are just like the counterpart functions
383without the _attrs suffixes, except that they pass an optional
384struct dma_attrs*.
385 386struct dma_attrs encapsulates a set of "dma attributes". For the
387definition of struct dma_attrs see linux/dma-attrs.h.
388 389The interpretation of dma attributes is architecture-specific, and
390each attribute should be documented in Documentation/DMA-attributes.txt.
391 392If struct dma_attrs* is NULL, the semantics of each of these
393functions is identical to those of the corresponding function
394without the _attrs suffix. As a result dma_map_single_attrs()
395can generally replace dma_map_single(), etc.
396 397As an example of the use of the *_attrs functions, here's how
398you could pass an attribute DMA_ATTR_FOO when mapping memory
399for DMA:
400 401#include <linux/dma-attrs.h>
402/* DMA_ATTR_FOO should be defined in linux/dma-attrs.h and
403 * documented in Documentation/DMA-attributes.txt */
404...
405 406 DEFINE_DMA_ATTRS(attrs);
407 dma_set_attr(DMA_ATTR_FOO, &attrs);
408 ....
409 n = dma_map_sg_attrs(dev, sg, nents, DMA_TO_DEVICE, &attr);
410 ....
411 412Architectures that care about DMA_ATTR_FOO would check for its
413presence in their implementations of the mapping and unmapping
414routines, e.g.:
415 416void whizco_dma_map_sg_attrs(struct device *dev, dma_addr_t dma_addr,
417 size_t size, enum dma_data_direction dir,
418 struct dma_attrs *attrs)
419{
420 ....
421 int foo = dma_get_attr(DMA_ATTR_FOO, attrs);
422 ....
423 if (foo)
424 /* twizzle the frobnozzle */
425 ....
426 427 428Part II - Advanced dma_ usage
429-----------------------------
430 431Warning: These pieces of the DMA API should not be used in the
432majority of cases, since they cater for unlikely corner cases that
433don't belong in usual drivers.
434 435If you don't understand how cache line coherency works between a
436processor and an I/O device, you should not be using this part of the
437API at all.
438 439void *
440dma_alloc_noncoherent(struct device *dev, size_t size,
441 dma_addr_t *dma_handle, gfp_t flag)
442 443Identical to dma_alloc_coherent() except that the platform will
444choose to return either consistent or non-consistent memory as it sees
445fit. By using this API, you are guaranteeing to the platform that you
446have all the correct and necessary sync points for this memory in the
447driver should it choose to return non-consistent memory.
448 449Note: where the platform can return consistent memory, it will
450guarantee that the sync points become nops.
451 452Warning: Handling non-consistent memory is a real pain. You should
453only ever use this API if you positively know your driver will be
454required to work on one of the rare (usually non-PCI) architectures
455that simply cannot make consistent memory.
456 457void
458dma_free_noncoherent(struct device *dev, size_t size, void *cpu_addr,
459 dma_addr_t dma_handle)
460 461Free memory allocated by the nonconsistent API. All parameters must
462be identical to those passed in (and returned by
463dma_alloc_noncoherent()).
464 465int
466dma_get_cache_alignment(void)
467 468Returns the processor cache alignment. This is the absolute minimum
469alignment *and* width that you must observe when either mapping
470memory or doing partial flushes.
471 472Notes: This API may return a number *larger* than the actual cache
473line, but it will guarantee that one or more cache lines fit exactly
474into the width returned by this call. It will also always be a power
475of two for easy alignment.
476 477void
478dma_cache_sync(struct device *dev, void *vaddr, size_t size,
479 enum dma_data_direction direction)
480 481Do a partial sync of memory that was allocated by
482dma_alloc_noncoherent(), starting at virtual address vaddr and
483continuing on for size. Again, you *must* observe the cache line
484boundaries when doing this.
485 486int
487dma_declare_coherent_memory(struct device *dev, dma_addr_t bus_addr,
488 dma_addr_t device_addr, size_t size, int
489 flags)
490 491Declare region of memory to be handed out by dma_alloc_coherent when
492it's asked for coherent memory for this device.
493 494bus_addr is the physical address to which the memory is currently
495assigned in the bus responding region (this will be used by the
496platform to perform the mapping).
497 498device_addr is the physical address the device needs to be programmed
499with actually to address this memory (this will be handed out as the
500dma_addr_t in dma_alloc_coherent()).
501 502size is the size of the area (must be multiples of PAGE_SIZE).
503 504flags can be or'd together and are:
505 506DMA_MEMORY_MAP - request that the memory returned from
507dma_alloc_coherent() be directly writable.
508 509DMA_MEMORY_IO - request that the memory returned from
510dma_alloc_coherent() be addressable using read/write/memcpy_toio etc.
511 512One or both of these flags must be present.
513 514DMA_MEMORY_INCLUDES_CHILDREN - make the declared memory be allocated by
515dma_alloc_coherent of any child devices of this one (for memory residing
516on a bridge).
517 518DMA_MEMORY_EXCLUSIVE - only allocate memory from the declared regions.
519Do not allow dma_alloc_coherent() to fall back to system memory when
520it's out of memory in the declared region.
521 522The return value will be either DMA_MEMORY_MAP or DMA_MEMORY_IO and
523must correspond to a passed in flag (i.e. no returning DMA_MEMORY_IO
524if only DMA_MEMORY_MAP were passed in) for success or zero for
525failure.
526 527Note, for DMA_MEMORY_IO returns, all subsequent memory returned by
528dma_alloc_coherent() may no longer be accessed directly, but instead
529must be accessed using the correct bus functions. If your driver
530isn't prepared to handle this contingency, it should not specify
531DMA_MEMORY_IO in the input flags.
532 533As a simplification for the platforms, only *one* such region of
534memory may be declared per device.
535 536For reasons of efficiency, most platforms choose to track the declared
537region only at the granularity of a page. For smaller allocations,
538you should use the dma_pool() API.
539 540void
541dma_release_declared_memory(struct device *dev)
542 543Remove the memory region previously declared from the system. This
544API performs *no* in-use checking for this region and will return
545unconditionally having removed all the required structures. It is the
546driver's job to ensure that no parts of this memory region are
547currently in use.
548 549void *
550dma_mark_declared_memory_occupied(struct device *dev,
551 dma_addr_t device_addr, size_t size)
552 553This is used to occupy specific regions of the declared space
554(dma_alloc_coherent() will hand out the first free region it finds).
555 556device_addr is the *device* address of the region requested.
557 558size is the size (and should be a page-sized multiple).
559 560The return value will be either a pointer to the processor virtual
561address of the memory, or an error (via PTR_ERR()) if any part of the
562region is occupied.
563 564Part III - Debug drivers use of the DMA-API
565-------------------------------------------
566 567The DMA-API as described above as some constraints. DMA addresses must be
568released with the corresponding function with the same size for example. With
569the advent of hardware IOMMUs it becomes more and more important that drivers
570do not violate those constraints. In the worst case such a violation can
571result in data corruption up to destroyed filesystems.
572 573To debug drivers and find bugs in the usage of the DMA-API checking code can
574be compiled into the kernel which will tell the developer about those
575violations. If your architecture supports it you can select the "Enable
576debugging of DMA-API usage" option in your kernel configuration. Enabling this
577option has a performance impact. Do not enable it in production kernels.
578 579If you boot the resulting kernel will contain code which does some bookkeeping
580about what DMA memory was allocated for which device. If this code detects an
581error it prints a warning message with some details into your kernel log. An
582example warning message may look like this:
583 584------------[ cut here ]------------
585WARNING: at /data2/repos/linux-2.6-iommu/lib/dma-debug.c:448
586 check_unmap+0x203/0x490()
587Hardware name:
588forcedeth 0000:00:08.0: DMA-API: device driver frees DMA memory with wrong
589 function [device address=0x00000000640444be] [size=66 bytes] [mapped as
590single] [unmapped as page]
591Modules linked in: nfsd exportfs bridge stp llc r8169
592Pid: 0, comm: swapper Tainted: G W 2.6.28-dmatest-09289-g8bb99c0 #1
593Call Trace:
594 <IRQ> [<ffffffff80240b22>] warn_slowpath+0xf2/0x130
595 [<ffffffff80647b70>] _spin_unlock+0x10/0x30
596 [<ffffffff80537e75>] usb_hcd_link_urb_to_ep+0x75/0xc0
597 [<ffffffff80647c22>] _spin_unlock_irqrestore+0x12/0x40
598 [<ffffffff8055347f>] ohci_urb_enqueue+0x19f/0x7c0
599 [<ffffffff80252f96>] queue_work+0x56/0x60
600 [<ffffffff80237e10>] enqueue_task_fair+0x20/0x50
601 [<ffffffff80539279>] usb_hcd_submit_urb+0x379/0xbc0
602 [<ffffffff803b78c3>] cpumask_next_and+0x23/0x40
603 [<ffffffff80235177>] find_busiest_group+0x207/0x8a0
604 [<ffffffff8064784f>] _spin_lock_irqsave+0x1f/0x50
605 [<ffffffff803c7ea3>] check_unmap+0x203/0x490
606 [<ffffffff803c8259>] debug_dma_unmap_page+0x49/0x50
607 [<ffffffff80485f26>] nv_tx_done_optimized+0xc6/0x2c0
608 [<ffffffff80486c13>] nv_nic_irq_optimized+0x73/0x2b0
609 [<ffffffff8026df84>] handle_IRQ_event+0x34/0x70
610 [<ffffffff8026ffe9>] handle_edge_irq+0xc9/0x150
611 [<ffffffff8020e3ab>] do_IRQ+0xcb/0x1c0
612 [<ffffffff8020c093>] ret_from_intr+0x0/0xa
613 <EOI> <4>---[ end trace f6435a98e2a38c0e ]---
614 615The driver developer can find the driver and the device including a stacktrace
616of the DMA-API call which caused this warning.
617 618Per default only the first error will result in a warning message. All other
619errors will only silently counted. This limitation exist to prevent the code
620from flooding your kernel log. To support debugging a device driver this can
621be disabled via debugfs. See the debugfs interface documentation below for
622details.
623 624The debugfs directory for the DMA-API debugging code is called dma-api/. In
625this directory the following files can currently be found:
626 627 dma-api/all_errors This file contains a numeric value. If this
628 value is not equal to zero the debugging code
629 will print a warning for every error it finds
630 into the kernel log. Be careful with this
631 option, as it can easily flood your logs.
632 633 dma-api/disabled This read-only file contains the character 'Y'
634 if the debugging code is disabled. This can
635 happen when it runs out of memory or if it was
636 disabled at boot time
637 638 dma-api/error_count This file is read-only and shows the total
639 numbers of errors found.
640 641 dma-api/num_errors The number in this file shows how many
642 warnings will be printed to the kernel log
643 before it stops. This number is initialized to
644 one at system boot and be set by writing into
645 this file
646 647 dma-api/min_free_entries
648 This read-only file can be read to get the
649 minimum number of free dma_debug_entries the
650 allocator has ever seen. If this value goes
651 down to zero the code will disable itself
652 because it is not longer reliable.
653 654 dma-api/num_free_entries
655 The current number of free dma_debug_entries
656 in the allocator.
657 658 dma-api/driver-filter
659 You can write a name of a driver into this file
660 to limit the debug output to requests from that
661 particular driver. Write an empty string to
662 that file to disable the filter and see
663 all errors again.
664 665If you have this code compiled into your kernel it will be enabled by default.
666If you want to boot without the bookkeeping anyway you can provide
667'dma_debug=off' as a boot parameter. This will disable DMA-API debugging.
668Notice that you can not enable it again at runtime. You have to reboot to do
669so.
670 671If you want to see debug messages only for a special device driver you can
672specify the dma_debug_driver=<drivername> parameter. This will enable the
673driver filter at boot time. The debug code will only print errors for that
674driver afterwards. This filter can be disabled or changed later using debugfs.
675 676When the code disables itself at runtime this is most likely because it ran
677out of dma_debug_entries. These entries are preallocated at boot. The number
678of preallocated entries is defined per architecture. If it is too low for you
679boot with 'dma_debug_entries=<your_desired_number>' to overwrite the
680architectural default.
681 682void debug_dmap_mapping_error(struct device *dev, dma_addr_t dma_addr);
683 684dma-debug interface debug_dma_mapping_error() to debug drivers that fail
685to check dma mapping errors on addresses returned by dma_map_single() and
686dma_map_page() interfaces. This interface clears a flag set by
687debug_dma_map_page() to indicate that dma_mapping_error() has been called by
688the driver. When driver does unmap, debug_dma_unmap() checks the flag and if
689this flag is still set, prints warning message that includes call trace that
690leads up to the unmap. This interface can be called from dma_mapping_error()
691routines to enable dma mapping error check debugging.
692 693